I agree that in the modern age we live in there has been something to that effect. Historically too I think, just going off fiction (which is often rather well researched). But there are also a lot of counter-examples there. Perhaps the question isn't whether there is/has been some standard of "don't go after the leaders of a country" but rather when and why one might. Your point about having someone that can actually agree to the end of a conflict and tell their side to stop fighting could be valuable. At the same time the idea of disrupting all the command and control paths is highly valuable during a conflict - and if that is best accomplished by removing the leadership it might outweigh the value of having someone that can authoritatively issue "stop" orders. I don't know if this administration, or the Israelis, thought taking out Khamenei and his leadership council would accomplish that or not. It does seem that Iran has a reasonably robust political and military line of succession.  The other view that comes to mind is just the problem all governments have to deal with: factions. Whatever form of government is in place has to manage all the factions -- no autocrat has ever been purely independent and able to do whatever that person wanted. They all manage the competing interests of others to remain in power. Perhaps taking out the leadership of a country shift the factional equilibrium towards a more favorable position (could be a difficult calculation if domestic knowledge about factional relationships is weak).